Isopropanol odor improvement by azeotropic distillation



April 29, 1952 s. w. WILSON 2,595,116

' ISOPROPANOL ODOR IMPROVEMENT BY AZEOTROPIC DISTILLATION Filed Aug. 20, 1949 :2 5 3 O H at (0 T8 d O .J

FRACTIONATOR ETHER :1 CoNcaN-rzAT-o g Q 111 L0 T IT E 6 r- 2 1 if i 3 I? N m O) m T MO HEADS COLUMN Samue CJTZJTZson. {Inventorb5 W M Otter-hes Patented Apr. 2%, 1952 TENT OFFICE ISOPROPANOL ODOR IMPROVEMENT BY AZEOTROPIC DISTILLATION ration of Delaware Application August 20, 1949,"Serial No. 111,357

3 Claims.

This invention relates to the purification of isopropyl alcohol and particularly to the production of an isopropyl alcohol of improved odor quality. Specifically, the invention relates to a process whereby malodcrous isopropyl alcohol is treated to remove the malodorous impurities therefrom.

It is well known that alcohols, particularly those produced by the acid-catalyzed hydration of olefin hydrocarbons, possess a distinct and apparent foreign odor, slightly penetrating and for the most part disagreeable. While no attempt will be made to definitely assign the disagreeable odor of alcohols prepared by olefin hydration'to the presence of any one or combination of chemical compounds, it can be said with reasonable assurance that the odor of crude alcohol depends to a large extent on the quality of the olefin stream employed in the olefin hydration operation. Likewise, the odor of a refined alcohol depends to a large extent on the quality of the crude alcohol from which it is prepared.

Olefin hydrocarbons, such as those produced by the cracking of mineral oils, contain variable amounts of compounds having an obnoxious odor particularly sulfur compounds as hydrogen sulfide, alkyl sulfides and mercaptans. These materials present, even in minute amounts,'in the olefin stream to the acid catalyzed hydration process are beli ved to contribute to the obnoxious odor of the crude alcohol since, while sulfur alone has no odor, it is clear that in combination with other elements it is a powerful odoriferous agent. The bad odor of alcohols has also been attributed to the presence of so-called high-boilingpolymer products formed in side reactions during the alcohol process. The odor of the polymer products is strengthened by the presence of any sulfur compounds dissolved therein, although the odor of some of the pure polymers themselves is by no means pleasing to the olfactory sense. A typical analysis of a sample of the so-called polymer product, in this case the propyl oil, resulting from the production of isopropyl alcohol by the sulfuric acid catalyzed hydration of propylene, is as follows:

55 weight per cent sec-heptanol (B. P. 137- 140 -2 21 weight per cent sec-octanol (B. P. -165 C.) 2 weight per cent C7 keto'ne (B. P. 131 C'.) I 13 weight per cent hydrocarbons (B. P. above C.) 9 weight per cent ethers (3.1 above 160 'C.) Traces of sulfur compounds The composition of the propyl oil obtained during the concentration ofdilute crude isopropa'nol varies according to thepoint from which the alcohol containing it is withdrawn in the concentrating tower, and the odor likewise varies. Cuts can be identified with an odor of camphor, of menth'oL-etc. It has also been reported that the presence of nitrogen compounds also contributes to the odor of alcohols.

The odor imparted to isopropyl alcohol by the propyl oil impurities is -a hydrocarbon type odor. This type of odor has been found to "be efiectively removed by a number of methods such as by careful and repeated conventional fractionations but particularly by the water extractive distillation method as described and claimed in U. S. patent application Serial No. 24,626, filed May 1, 1948, and assigned to the present assignee. However, there is a second type of odor which is fugitive in nature and which may best be characterized as a mercaptan odor, and which is attributed to the presence of low-boiling sulfur compounds such as mercaptans, thioethers or thioaldehydes which appear to be decomposition products of higher-boiling impurities which break up under the action of heat during the alcohol purification distillation operations. It has been foundthat this second type of odor (hereinafter referred to as mercaptan odor) is particularly noticeable'when the alcohol production process is operated under upset regenerator conditions or when the purification process is operated under upset'concentrator conditions. When the'capacity of theregenerator or concentrator is exceeded some isopropyl alcohol creeps down into the reboiler associated with the regeneratoror concentrator wherein a high temperature prevails and where a'particular concentration of sulfuric acid exists. Under these conditions it is felt that'the sulfuric acid'acts as a reducing agent on the isopropyl alcohol which decomposes'into lower-boiling impurities, particularly mercaptan and thioether compounds which are volatile. These impurities go overhead with the alcohol from the regenerator. Subsequently in the concentrator they are not all removed with the low boiling overhead, but remain in traces of parts per million in the isopropyl alcohol product which is removed as a top sidestream from the concentrator. Likewise if the concentrator conditions are upset the same decomposition occurs and the problem becomes aggravated. It has also been theorized that the second type of odor is due to the decomposition of sulfurized esters which are formed in minute amounts during the propylene absorption in sulfuric acid. These esters are thought to decompose either thermally or by hydrolysis.

Furthermore, there are indications that some of the malodorous compounds are rather strongly bound in the aqueous alcohol product in such a manner that they tend to concentrate along with the alcohol during such operations as heads removal, water extractive distillation and even straight fractionation for alcohol finishings. However, the nature of the chemical reactions tending to bind these malodorous compounds is such that unstable addition compounds result and the malodorous compounds are liberated and volatilized to some extent during all alcohol finishing operations of the types described. For instance, in typical alcohol finishing operations the following problems are encountered. Crude alcohol, after passing through the heads operation to remove ether and other low-boilers, is fed to a finishing column in which certain low-boiling materials are taken overhead with a purified alcohol taken as a side stream or bottoms product. Materials contributing to recycle odor appear to be lower boiling than the alcohol proper but are nevertheless not completely removed from the alcohol sidestream. This result is believed to be due in large part to the fact that decomposition reactions liberating low-boiling malodorous compounds occur during the distillation, allowing these malodorous compounds to pass overhead. Similar difiiculties are encountered when applying water extractive distillation to the same crude alcohol; In fact, water extractive distillation appears to concentrate the compounds giving rise to mercaptan odor along with the alcohol so that, in the final alcohol finishing operation, an alcohol product high in mercaptan odor is obtained. This seems to be particularly true of isopropyl alcohol prepared by the so-called weak acid method as described below.

It has beenfound that isopropyl alcohol of improved odor characteristics and particularly an isopropyl alcohol substantially free of mercaptan odor can be produced by the addition of a small amount of a low-boiling saturated aliphatic ether, say 0.1% to by volume, to the bottoms from the head column or to an off-odor 91% isopropanol and subsequently distilling the same to remove the ether and other low-boiling materials from the alcohol.

The mechanism by which this method has produced such improved results is not understood, however, one theory is postulated as follows: The alcohol concentrating column is operated at a higher temperature than the heads column. There is a quantity of high-boiling impurities in the alcohol bottoms from the heads column. When this alcohol containing the high-boiling impurities is distilled in the concentrating column for the removal of water therefrom, the highboiling compounds, being subjected to the higher temperatures prevailing therein, decompose into lower-boiling malodorous compounds which the ether entrains and removes. Since the impurities giving rise to mercaptan odor are present only as trace impurities, it is extremely difficult if not impossible to locate and identify any cognizable impurities from the distillation operation.

In carrying out the invention it is preferred to employ diisopropyl ether since this ether is produced in the alcohol process and is readily available. However, diethyl ether has produced the same results and other saturated aliphatic ethers boiling below isopropanol are suitable. As little as 0.1 volume per cent ether has been found to improve the alcohol quality but it is preferable to use about 0.5-5 volume per cent although amounts up to about 10% by volume may be used if the crude is of particularly bad mercaptan odor. Generally 1% by volume of ether has been found satisfactory to remove the mercaptan odor present in the ordinary commercial isopropanol product.

Crude isopropyl alcohol may be produced by the sulfuric acid catalyzed hydration of propylene by the weak acid method or the strong acid method. In the former process a relatively rich propylene stream containing about -80 volume per cent propylene is absorbed in sulfuric acid of approximately to 80 weight per cent concentrations, preferably weight per cent at about 170 F. and approximately 250 p. s. i. g. pressure to form an extract comprising isopropyl sulfate which is partially hydrolyzed to alcohol during the absorption. The hydrolysis is completed by dilution of the extract with water to approximately 45 weight per cent acid strength and maintaining the extract at a temperature of about 190 F. for a period of about 10 minutes residence time. Th hydrolysis products are then distilled in an alcohol regenerator wherein crude alcohol vapors are removed overhead and condensed, and wherein spent sulfuric acid is recovered as bottoms for reconcentration and subsequent re-use in the absorption process. The crude isopropyl alcohol contains approximately 30-60% by volume of isopropyl alcohol, some impurities including hydrocarbons, isopropyl ether, acetone and unknown impurities, and the balance water. The crude alcohol is condensed and cooled and is ready for the purification stage of the process.

In the strong acid process a propylene stream of relatively lower propylene content, i. e., about 30-60 volume per cent propylene, is contacted under similar conditions with a stronger sulfuric acid, e. g., acid of -95 weight per cent concentration, preferably -88 weight per cent, whereby an extract is formed as above. The extract goes through a similar hydrolysis and stripping treatment to produce a crude isopropyl alcohol as described for the weak-acid process.

When the isopropyl alcohol is purified by conventional fractionation it is led to a heads column the purpose of which is to remove ether and other low-boiling, water-insoluble impurities such as hydrocarbons, hydrocarbon polymers, oxygenated compounds, and unknown impurities. This is accomplished by a steam stripping operation whereby the lighter materials are withdrawn overhead while Weak aqueous alcohol is recovered as bottoms. The weak aqueous alcohol is then distilled in a concentrating column where the alcohol is concentrated to a composition approaching its water azeotrope, viz., 91 volume Isopropanol 90+ volume per cent Diisopropyl ether 5-10 volume per cent Acetone Hydrocarbons (B. P. 60-300" C., major portion l-300 C.) oxygenated compounds other "than Up to 2 acetone incl. higher ethers, t-butyl volume and higher alcohols, higher ketones, per cent etc. of wide boiling range Traces of sulfur compounds, boiling over a wide range The attached drawing illustrates in-diagrammatical view one arrangement of apparatus for carrying out the present invention.

Referring to the drawing crude isopropanol of a composition as. set forth above is introduced via line I into a heads column 2. Sufiicient heat is applied to column 2 to distill overhead as completely as possible the diisopropyl ether, the lowboiling oxy-compounds and the low-boiling hydrocarbons, the same being removed via line 3. Some alcohol is also taken overhead to insure more complete removal of the low-boiling materials. To recover this alcohol the overhead is passed via line 3 to wash vessel 4 wherein the total overhead is washed with water introduced via line 5. An ether layer is removed via line 5 and dilute alcohol is returned to the heads column from the wash vessel via line I. Aqueous alcohol substantially free of the light low-boiling impurities but containing some high-boiling impurities is removed from a point near the bottom of the heads column via line 8. Ether is added in the designated amount to the aqueous alcohol via line 9, preferably before the introduction of the aqueous alcohol to the concentrating column l0. At this point it should be stated that the ether is added in this manner since it is not practical to attempt to control the ether content of the heads column bottoms by adjusting the ether content of the overhead. In a continuous operation the ether content of the crude isopropanol varies, and in addition the heads column also must operate to most efiiciently remove substantially all the low-boiling oxy-compounds and hydrocarbons from the alcohol.

In the concentrating column distillation is brought about to remove overhead via line I I the ether and all the alcohol with the accompanying azeotropic amounts of water. Water is removed from the system via line l2. High-boiling impurities are preferably removed from column H) as a sidestream via lines IE or ill at a point above or below the alcohol feed plate to the column. The overhead from column I9 is subjected to a final distillation in column l3 wherein ether, acetone, malodorous low-boiling decomposition products and small amounts of isopropanol are removed overhead via line [4, while the purified isopropanol of good odor characteristics is removed as a bottoms product via line l5.

Althoughit is preferred to remove the malodorous bodies from the isopropanol in the above \manner it is also possible to improve the odor by adding the ether to the 91% isopropanol'azeotrope and distilling the same as in the manner set forth in column I3. In this variation ether would be added via line I! rather than line 9.

Data in Table I demonstrates the effective use of diisopropyl ether in the removal of mercaptan odor from isopropanol. In the runs set forth in Table I the ether was added to the'bottoms from the heads column. In the heads column a crude. isopropanol prepared by the sulfuric acid catalyzed hydration of propylene was distilled inorder to remove ether, acetone, hydrocarbons and other low-boiling materials as substantially completely as possible. The bottoms from the heads column containing alcohol, water and high-boilers together with any unremoved low-boilers was then distilled in a concentrating column with and without the addition of 0.5 volume percent diisopropyl ether. The original alcohol had a Grade 4 mercaptan odor. Mercaptan odors were graded 1 to 4, the higher the number the greater the mercaptan odor. The concentrating column employed was a Fenske column of the indicated dimensions with a packing of stainless steel helices and about 30 theoretical plates. The mixture was distilled until all the alcohol and material boiling above alcohol was distilled overhead. The overhead was then subjected to another fractionation step for the removal of the low-boiling impurities originally present and low boiling decomposition products. Ten cuts of alcohols were removed as sidestreams and examined for mercaptan odor. The results are compared below indicating that the mercaptan odor still remained in the finished alcohol where ether was not added to the heads column bottoms prior to the distillation, whereas in the other runs wherein ether was added even in the small amounts employed, the mercaptan odor was completely removed.

Table I Column 3l ID 51 ID 5l" ID Reflux Ratio 10/1 5/l.. 5/1. Vol. Heads Btms. Charged. cc. 500 500 500. VoLDiisopropylEthcr Chgd..cc 2.5 (0.5vol none 2.5 (0.5 vol per cent). per cent) Odor of Plant Product 4 M0 4 M0 M0. Laboratory Finished Produ t' Cut #1 NMO 4M0 NMO. Cut #2. NMO 4M0 NMO. Out #3.... NMO 4M0 MO Cut #4.. NMO..-. 4M0 NMO Cut #5.. NMO 4M0 NMO. Cut #6.. NMO 4 MO. NMO Cut #7.. NMO 4M0 NMO. Cut #8.. NMO 4 MO NMO. Cut #9 NMO 4M0 NMO. Cut #10 NMO. 4M0 NMO.

1 MO=mercaptan odor. Rated 1 to 5, the higher the figure the more pronounced the mercaptan odor.

2 MO=N0 mercaptan odor.

Data in Table II demonstrates that the ether is also efiective in removing odor when added to the 91 volume percent isopropanol azeotrope rather than to the heads column bottoms. In the runs tabulated in Table II a top stream was removed containing the added ether and associated low-boiling impurities giving rise to the bad odor. In the final run wherein no ether was added the alcohol was distilled and low-boilers were removed in the first two cuts. Mercaptan odor continued to permeate the next 6 outs.

Table II Column 3-1"ID 31" ID 5-1"ID 31" ID 3-1" ID 5'%" ID RetluxRatio 5/1 7/1".-- 5/l. 5/1"---" 1011..-- 15/1. Vol. Alcohol Ghargeduiuccn 500 250 250 250 250 500. Odor of Alcohol Charged 4MO. 4MO 4MO 4MO 4MO 4M0.

diisopropyl. VoL/Etlier Charged cc 45(9 vol. 2.5 (lvol. 2.5 (1 vol. 2.5(1v01. 2.5 (lvol. None.

per cent). per cent). per cent). per cent). per cent). Alcohol Odor aftcr Acetone and Ether Removal:

Cut#1 NMO."- NMO.- NMO NMO.--. NMO.-" NMO. Cut#2 NMO.." NMO.. NMO---. NMO. NMO. Cutt3-.- NMO.." NMO NMO NMO. 4M0 Cuti- NMQM NMO.." NM NMO. 4M0 Cut#5. NMO. NMO NM NMO, 4M0 Cut#6. NM NMO NMO.. NMO.--. 4M0 Cut#7- NMO.. NMO..-. NMO..-- NMO.--. 4M0 Cutis.-- NMO NMO NMO.. NMO 4M0. Cut#9 1 NMO NMO NMO NM NMO. Cut#l0 -1 NMO..-. NMO..-- NMOHH NM NMO NMO 1\10=mercaptan odor NMO=no mercaptan odor What is claimed is:

1. A process for the production of isopropanol of improved odor quality from aqueous isopropanol containing low-boiling and high-boiling impurities including malodorous bodies imparting mercaptan odor to the isopropanol, which comprises distilling the aqueous isopropanol to remove as a first distillate the low-boiling impur 2. A process according to claim 1 in which 0.5 to 5.0 volume percent of diisopropyl ether is added to the distillation bottoms.

3. A process according to claim 1 in which 0.5 to 5.0 volume percent of diethyl ether is added to the distillation bottoms.

SAMUEL W. WILSON.

REFERENCES CITED The following references are of record in the file of this patent: 4

UNITED STATES PATENTS Number Name Date 2,140,694 Evans Dec. 20, 1938 2,309,653 Leum et al Feb. 2, 1943 2,351,527 Lembcke June 13, 1944 2,481,211 Fuqua Sept. 6, 1949 2,487,086 Amick, J12, et a1. Nov. 8, 1949 2,489,619 Carlson et a1. Nov. 29, 1949 

1. A PROCESS FOR THE PRODUCTION OF ISOPROPANOL OF IMPROVED ODOR QUALITY FROM AQUEOUS ISOPROPANOL CONTAINING LOW-BOILING AND HIGH-BOILING IMPURITIES INCLUDING MALODOROUS BODIES IMPARTING "MERCAPTAN" ODOR TO THE ISOPROPANOL, WHICH COMPRISES DISTILLING THE AQUEOUS ISOPROPANOL TO REMOVE AS A FIRST DISTILLATE THE LOW-BOILING IMPURITIES LEAVING AQUEOUS ISOPROPANOL CONTAINING HIGHBOILING IMPURITIES AS BOTTOMS, ADDING TO THE BOTTOMS 0.1 TO 10.0 VOLUME PERCENT OF A SATURATED ALIPHATIC ETHER BOILING BELOW ISOPROPANOL, DISTILLING THE BOTTOMS TO REMOVE AS A SECOND DISTILLATE THE ETHER, SUBSTANTIALLY ALL THE ISOPROPANOL WITH AZEOTROPING AMOUNTS OF WATER AND THE MALODOROUS BODIES, LEAVING THE HIGH-BOILING IMPURITIES AND THE REMAINING WATER AS A RESIDUE, AND FRACTIONALLY DISTILLING THE SECOND DISTILLATE TO REMOVE THE ETHER AND MALODOROUS BODIES AS A THIRD DISTILLATE FROM THE ISOPROPANOL OF IMPROVED ODOR QUALITY. 